The invention relates to an apparatus and technique for aligning a test surface or test beam in an infrared interferometer to an optical axis of the interferometer.
U.S. Pat. No. 4,201,473 (Domenicali et al.) issued May 6, 1980 describes an interferometer in which a diffuse screen has thereon an integral alignment reticle physically located at the focus of wavefronts reflected by test and reference surfaces to be aligned with an optical axis of the interferometer. During alignment of commercial embodiments of this interferometer, the optics are configured so that a spot of light which represents the image of the test surface to be aligned is focused onto the diffuse screen. The surface of the diffuse screen itself is uniformly illuminated so that the reticle thereon can be viewed. The spot appearing on the diffuse screen then is imaged onto the lens of a video camera together with an image of the illuminated diffuse screen and the opaque reticle thereon. The spot produced by the test surface to be aligned, the surface of the diffuse screen and the reticle are viewed on a video monitor connected to the output of the camera. When the test surface has been adjusted so that the monitor image of the spot is aligned with the monitor image at the marked center of the reticle, the test surface is aligned. A part of the optics including the diffuse screen then is switched out of the path to the lens of the video camera, so the video camera can be used to view an interference pattern produced by beams reflected from a reference surface and the test surface.
The assignee of the present application has utilized a different alignment system in a product known as the WYKO 6000. This alignment system contains no integral reticle or any other physical indicia anywhere on the optical axis of the interferometer. Instead, an electronically-generated alignment indicia is produced at the surface of a video monitor. No integral reticle is provided on a diffuse screen, but spot images of test and reference surface are imaged onto a diffuse screen. The diffuse screen is not illuminated and therefore cannot be viewed on the video monitor. The spot images on the diffuse screen are viewed on a video camera. The color of the software-generated alignment indicia on the monitor can be different than the imaged spots as they appeared on the monitor screen, and are essentially "transparent" so that the spot images appear clearly even if located "behind" a part of the software-generated alignment indicia.
Typical alignment procedures, such as the one described in the above Domenicali patent and the Wyko 6000 interferometer, are problematic for use in infrared interferometers because infrared video cameras usually contain Vidicon tubes that are very sensitive to heat. The energy levels associated with the infrared laser typically are many watts, and a focused infrared laser beam is capable of reaching temperatures over 370 degrees Centigrade in time periods of much less than one second. If an infrared beam impinging on a Vidicon tube increases a temperature therein above 49 degrees Centigrade, irreparable damage to material of a surface of the Vidicon tube is caused, and the Vidicon tube must be replaced or repaired at a very high cost. Since an infrared laser beam generates heat almost instantaneously on a surface on which the beam impinges, alignment techniques for infrared interferometers cannot include focusing an infrared beam directly on an infrared camera. Therefore helping alignment of a test beam in an infrared interferometer is important because the infrared laser beams are invisible, unlike the situation described above in the Domenicali patent and in the Wyko 6000 system.